Head pressure is calculated and represented in terms of feet (ft.). In order to calculate the total head pressure of your application, you'll need to know the difference in elevation between your return pump and return outlet. You will also need to know the total number and type of fittings used as well as the total distance of pipe used.
The effect of gravity on head pressure is very simple. Every vertical foot of distance the return pump moves water equates to 1 foot of head pressure. The effects of friction on head pressure are more difficult to calculate. Roughly every 10 feet of pipe through which water is traveling adds 1 foot of head pressure. Additionally, every 90-degree bend adds 1 foot of head pressure.
Pipe diameter is also an extremely important factor when calculating head pressure. As a general rule of thumb, reducing the size of pipe below the return pump output will drastically increase head pressure. For minimum head pressure, using the largest diameter pipe possible is best. Avoiding things like flow meters, reactors and chillers will also help to reduce head pressure.